Our VN750 has Automatic Cam Chain Tensioners (ACCT) that have weak springs in
them.
The tensioner is a basic design, The main parts is a spring, slotted screw
and a threaded plunger. How it works is, one end of the spring goes in the
slotted screw and the other end of the spring goes into a groove in the
tensioner housing. The spring is preloaded by turning the screw clockwise 4-5
times. The plunger is then screwed on a couple threads then held in place by a
retainer. The spring is now constantly trying to unscrew (turn CCW) the slotted
screw out of the plunger. This in turn keeps the plunger always wanting to be
extended. This is what makes the tensioner give constant pressure on the cam
chain automatically. The plunger has a mushroomed end which presses on the
assembly inside the engine which goes in contact with the cam chain itself. You
can see now that if the spring is weak the pressure the tensioner gives the cam
chain will also be weak.
NOTE: The plunger will not retract simply by pushing in on it. You will
need to turn the screw CW at the same time. This is good because the cam chain
can't push it in either. If it could it would be a shock absorber instead of a
tensioner. This is good to know when you are installing the tensioner back in
the engine.
When the tensioner no longer can keep the cam chain at the proper tension
the cam chain becomes loose. You will notice several problems with your engine
when this begins to happen. Most of the noise you hear may only appear when the
engine is cold but it may go away once the engine warms up. It will be a chain
slapping sound from one of your cylinders. Because a loose chain will affect
your valve train timing you may also hear an engine knock. It's as if you have
bad gas. Another problem my bike had when this happened to me was I would get
raw gas coming out of that cylinders exhaust. I know now that my exhaust valve
must been opening during compression because the valves timing was off.
The cause of your tensioner to stop working as designed can either be
because the tensioner is stuck in a loose position or the spring no longer has
the proper tension.
I'm always one to do the easier thing first so try this first. You can "unstick"
a stuck tensioner without even removing it form the bike. This is called the "Grambo
Trick".
NOTE: If you do this with the engine running make sure you don't turn it
too much clockwise. This will loosen the tension on the cam chain and may
make your chain jump the sprocket.

GRAMBO TRICK:

You just take the little bolt off the top of the tensioner housing.
Then turn the little slotted bolt inside. You can feel one way
(clockwise) retracts it against the spring. Let go and it will spin
back in. The idea is that they sometimes get "stuck", so after it
spins back down, try to turn it a bit more in the CCW direction.
Dunno if it's the plunger itself that sticks, or the chain guide
inside that needs a bit more pressure than the spring allows
to "unstick". Anyway, this is a no-risk process and some members
have said they've gone thousands of noise-free miles after doing
this.

If that doesn't solve the problem then you need to reload the weak spring in the
tensioner. Gypsy has great instruction at her website on how to do this. Several
of us have used these instructions with great success. You can get those
instructions at this link:

By using Gypsy's instructions and James' pictures you will find that you will be
able to do this fix yourself. If there is anything you don't understand you can
post the question to the group and one of us who have done this will be able to
help you through it.